There are many devices for precisely determining a displacement and a speed of a movable member for feed-back control. One example of such a device includes a lens barrel of an autofocus camera. In the lens barrel, a focusing mechanism for advancing and retreating a focus lens, using an electric motor or a supersonic motor, is provided. In order to determine a rotation displacement of a rotating barrel constituting the focusing mechanism, a magnetic encoder is used. Patent Document 1 discloses a magnetic encoder used in such a focusing mechanism, of which outside perspective view is shown in FIG. 17. According to the technique disclosed in Patent Document 1, a magnetic sensor 405″ is pressed onto a magnetic medium 415 having a curvature provided along the lens barrel 410. The magnetic sensor 405″ includes a magnetic sensor element 401 and a pressure spring 402. As seen from FIG. 17, the thickness of the magnetic sensor 405″ is very small, compared to the size of a part thereof opposite to a magnetic medium. As a thin magnetic sensor is required in the limited space in the barrel, a magnetic encoder is more often used than an optical encoder, which cannot be readily formed thin. With this focusing mechanism, an output from the magnetic sensor element 401 is fed back to drive the motor 411 to focus.
For highly accurate determination of a displacement, high resolution is required to a magnetic encoder. Resolution can be expressed by a magnetizing pitch of a magnetic medium. Although a conventional magnetizing pitch is of 30 to 50 μm, a magnetizing pitch of 10 to 20 μm, or even 10 μm or smaller, is recently required. However, with a tendency of higher resolution, the interval, or a gap, between the magnetic medium and the magnetic sensor element comes to be more influential. In view of the above, gap variation needs to be prevented. For this purpose, a method for sliding the magnetic medium and the magnetic sensor element placed in contact with each other is advantageous and often employed.
In the following, for readily understandable explanation of a positional relationship between a magnetic sensor and a magnetic medium, a relative movement direction of the magnetic medium and the magnetic sensor is defined as the X axis. In the case of a magnetic medium having a curved surface, the tangent direction of the magnetic medium at a point where the magnetic sensor contacts the magnetic medium is defined as the X axis.
Further, one of the directions perpendicular to the X axis, a direction intersecting the curvature center of the magnetic medium (a radius vector direction) is defined as the Z axis, and the other as the Y axis. Still further, when a point where the magnetic sensor is pressed onto the magnetic medium (a pressure point) is located on the Z axis, the point is defined as the origin of the X axis, and a displacement in the X direction from the X axis origin is defined as an X offset. Yet further, for more understandable explanation of a relative posture of the sliding surface of the magnetic sensor and the magnetic medium, an angle at which the sliding surface rotates around the X axis being as a rotation axis is referred to as a pitch angle. Still further, an angle at which the sliding surface rotates around the Y axis being as a rotation axis is referred to a roll angle. The roll angle with the sliding surface in parallel to the X axis is defined as 0 degrees. The pitch angle with the sliding surface in parallel to the Y axis is defined as 0 degree.
FIG. 18 shows a structure of the pressure spring 402. The pressure spring 402 urges the magnetic sensor element 401 substantially uniformly onto the magnetic medium 415 in assembling to thereby keep the pitch angle of the magnetic sensor stable in sliding. The magnetic sensor element 401 is attached to a holder 406, and the holder 406 can rock relative to the pressure spring 402 with a rocking central axis defined on the back surface of the holder 406 as a fulcrum. With the above, even though the distance between the fixing portion of the pressure spring 402 and the magnetic medium should vary, the magnetic sensor element 401 can be kept closely attached to the magnetic medium 415 via a spacer 407 as the holder rocks relative to the pressure spring 402. As the magnetic sensor element rocks with the rocking central axis in substantially parallel to the displacement direction of the magnetic medium as a fulcrum, the magnetic sensor element remains closely attached to the magnetic medium via a spacer or the like in-between, and the amount of movement of the magnetic medium (that is, the amount of advancement and retreatment of the focus lens) can be determined with high accuracy. The rocking center functions as a fulcrum for rocking, constituting a pressure point 408 at which the magnetic sensor element 401 is pressed onto the magnetic medium 415. An output of the magnetic sensor element 401 is extracted using an FPC (Flexible Print Circuit) 412.
Patent Document 2 describes a structure in which one leaf spring bears a pressure function and a function for keeping a constant pitch angle. As shown in FIG. 19, the leaf spring 531 holds a magnetic sensor element 501 by a sensor holding portion 524, and supports the sensor holding portion 524 by a first arm portion 555, a connection portion 556, and a second arm portion 557, and a fixing portion 526 is fixed to a mount pedestal 523. An output from the magnetic sensor element 501 is extracted using an FPC 512. As shown in FIG. 20, even though the distance between the fixing portion 526 of the leaf spring 531 and the magnetic medium 515 should vary, the first arm portion 555 and the second arm portion 557 flex in the opposite directions from each other, whereby the pitch angle can be kept constant. However, with a request for further accuracy, gap variation due to an X offset of the magnetic medium 515 and the magnetic sensor element 501 and the roll angle comes to be a larger problem to be solved.
Patent Document 3 discloses a method for reducing gap variation due to an X offset of the magnetic sensor element 601 and the roll angle. As shown in FIG. 21, the magnetic sensor element 601 of which width w in the slide direction is very narrow, that is, twice to fifteen times the magnetizing pitch (0.04 to 0.3 mm), is proposed. According to this technique disclosed in Patent Document 3, by defining small, that is, 0.3 mm or smaller, the width in the slide direction of the magnetic sensor element 601 that contacts the magnetic medium 615, gap variation due to an x offset and the roll angle variation is reduced, whereby the signal output amplitude is stabilized.
Patent Document 4 discloses leaf spring members having spring arm portions for connecting a fixing portion and a fixedly attached sensor portion to each other, extending in the respective returned directions, a sensor support mechanism using the same, and a rotary encoder.
Patent Document 5 discloses a magnetic encoder having a magnetic sensor holding mechanism for connecting two sides of a fixing portion and four sides of a sensor holding portion by four elastic arm portions, as shown in FIG. 22.